Method and system for evaluating an object

ABSTRACT

A method, system and a computer program product for evaluating a object; the method includes: (i) obtaining an image of an area of the object; wherein the area comprises multiple arrays of repetitive structural elements that are at least partially surrounded by at least one group of non-repetitive regions; wherein non-repetitive regions that belong to a single group of non-repetitive regions are ideally identical to each other; wherein the non-repetitive regions are arranged in a repetitive manner; and (ii) providing an evaluation result in response to a comparison between image information of a first sub-area to image information of a second sub-area that is proximate to the first sub-area; wherein the first sub-area comprises a first array of repetitive structural elements and a first non-repetitive region; wherein the second sub-area comprises a second array of repetitive structural elements and a second non-repetitive region.

FIELD OF THE INVENTION

This invention is generally in the field of automatic inspection ofobjects such as wafers, masks or reticles for micro-fabrication, flatpanel displays, micro-electromechanical (MEMs) devices and the like.

BACKGROUND OF THE INVENTION

Over the past decade, defect inspection to detect microscopicmanufacturing defects has become a standard part of micro-fabricationmanufacturing flows, especially for semiconductor wafers.

Each type of inspection technology is usually applied at steps in thesemiconductor manufacturing flow where it is best suited to the types ofdefects most likely to be found. The economic benefits of inspectionhave been substantial and inspection is generally accepted as havingmade a significant contribution to the substantial increase insemiconductor wafer manufacturing yields seen in the 1990s.

Inspection systems are employed in a number of different applicationsincluding: process monitoring to flag when a particular process step inthe manufacturing flow has an increased defect density above the levelnormally expected at that step; problem solving by inspecting so-calledshort-loop wafers that have only been processed with a subset of themanufacturing process steps in order to facilitate troubleshooting anddiagnosis or optimization of a particular subset of process steps andduring process development in order to optimize a new manufacturingprocess to reduce or eliminate process-specific or systematic defectmechanisms.

Wafer inspection systems for patterned wafer inspection usually work asfollows. A high powered microscope, traditionally an optical microscope,but more recently a SEM (Scanning Electron Microscope) or electronmicroscope, is set up under computer control to acquire sequentiallyimages of the area of objects such as a wafer that include multipleintegrated circuits that are arranged in dice, masks or reticles formicro-fabrication, flat panel displays, micro-electromechanical (MEMs)devices and the like during and after manufacture.

The image or contrast data that is acquired in this manner is mencompared to reference data. Defects are found or detected where thereare differences between the reference and the acquired images. Thereference images may be derived from Computer Aided Design data as isoften the case with mask or reticle inspection. Additionally oralternatively, the reference images may simply he images of neighboringcells or die on the wafer or similar wafer being inspected. Thesensitivity of the defect inspection process to small defects can becontrolled by adjusting the image acquisition parameters such as pixelsize, contrast, brightness, charging and bias conditions etc., and imageprocessing parameters that are used to compare the acquired inspectionimages and reference images.

An abject such as a wafer can include repetitive regions that includemany repetitive structural elements such as memory cells (such as SRAM,DRAM, FRAM, Flash memory). Repetitive regions can also includerepetitive structural elements programmable logic cells such as thoseincluded in PLAs, PLDs. Yet other objects such as MEMs displays and flatpanel displays can include repetitive regions.

Typically, ideally identical structural elements (referred to as cells)that are included within the same repetitive region are compared to eachother or to a so-called “golden cell”. Both types of comparison areknown as cell to cell comparison.

Memory arrays are often surrounded by non-repetitive regions. Thesenon-repetitive regions are commonly compared to each other using aso-called die to die comparison.

Die to die comparison involves comparing image information of one die toimage information of another die. It is known in the art that die to diecomparison is significantly less sensitive to defects due to its reducedsensitivity in relation to cell to cell comparison. The latter hasadvantages over die to die comparison because it is less sensitive toprocess variations, color variations (a phenomena that occurs whenbright field technology is applied and when a partially transparentlayer is positioned above the cells), and to changes in the opticalimage acquisition (and/or illumination) process. These changes caninclude radiation intensity fluctuations, optics aberrations; focusrelated inaccuracies, sensor saturation, sensor array on-uniformity,misalignment, and the like.

Even cell to cell comparison has some disadvantages. It is responsive todifferences between the acquisition of image information of cells thatare close to the edges of the repetitive regions and the acquisition ofimage information of cells that are spaced apart from the edges of therepetitive regions, it is noted that image information of a certainstructural element can be affected by the surrounding of that certainstructural element due to relatively wide energy distribution ofradiation that is used to illuminate the area and as a result of patterninformation that passes through spatial filters when illuminatingstructural elements that are near the edge of the array. Typically, suchspatial filters are adapted to block interference lobes of an array ofrepetitive structural elements. Non-repetitive regions can alter thelocation of the interference lobes thus pattern information can passthrough these spatial filters.

FIG. 1 illustrates area 8 of a prior art wafer. Multiple memory cellarrays 10-18 are surrounded by vertical stitches (denoted V) andhorizontal stitches (denoted “H stitch” 20-30). During a commonlyimplemented hybrid comparison process image information of repetitivestructural elements within each array is compared to image informationof repetitive structural elements within the same array while imageinformation of vertical stitches are compared to image information ofvertical stitches of another die. This hybrid comparison process suffersfrom the mentioned above drawbacks of die to die comparison and from thedrawbacks of cell to cell comparison.

There is a growing need to provide improved systems, methods andcomputer program products that can evaluate an object.

SUMMARY OF THE INVENTION

A method for evaluating a object; the method includes: (i) obtaining animage of an area of the object; wherein the area comprises multiplearrays of repetitive structural elements mat are at least partiallysurrounded by at least one group of non-repetitive regions; whereinnon-repetitive regions that belong to a single group of non-repetitiveregions are ideally identical to each other; wherein the non-repetitiveregions are arranged in a repetitive manner; and (ii) providing anevaluation result in response to a comparison between image informationof a first sub-area to image information of a second sub-area that isproximate to the first sub-area; wherein the first sub-area comprises afirst array of repetitive structural elements and a first non-repetitiveregion; wherein the second sub-area comprises a second array ofrepetitive structural elements and a second non-repetitive region.

A system for evaluating an object; tire system comprises: a memory unitadapted to store an image of an area of the object; wherein the areacomprises multiple arrays of repetitive structural elements that are atleast partially surrounded by at least one group of non-repetitiveregions; wherein non-repetitive regions that belong to a single group ofnon-repetitive regions are ideally identical to each other; wherein thenon-repetitive regions are arranged in a repetitive manner; and aprocessor adapted to provide an evaluation result in response to acomparison between image information of a first sub-area to imageinformation of a second sub-area that is proximate to the firstsub-area; wherein the first sub-area comprises a first array ofrepetitive structural elements and a first non-repetitive region;wherein the second sub-area comprises a second array of repetitivestructural elements and a second non-repetitive region.

A computer program product comprising a computer useable medium having acomputer readable program, wherein the computer readable program, whenexecuted on a computer, causes the computer to: receive an image of anarea of the object; wherein the area comprises multiple arrays ofrepetitive structural elements that are at least partially surrounded byat least one group of non-repetitive regions; wherein non-repetitiveregions that belong to a single group of non-repetitive regions areideally identical to each other; wherein the non-repetitive regions arearranged in a repetitive manner; and provide an evaluation result inresponse to a comparison between image information of a first sub-areato image information of a second sub-area that is proximate to the firstsub-area; wherein the first sub-area comprises a first array ofrepetitive structural elements and a first non-repetitive region;wherein the second sub-area comprises a second array of repetitivestructural elements and a second non-repetitive region.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, an embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 illustrates an area of a wafer;

FIG. 2 illustrates two sub-areas according to an embodiment of theinvention;

FIG. 3 illustrates two sub-areas according to another embodiment of theinvention;

FIG. 4 illustrates a method according to an embodiment of the invention;

FIG. 5 illustrates a method according to another embodiment of theinvention; and

FIG. 6 illustrates a system according to an embodiment of the invention.

DETAILED DESCRIPTION

A method, system and computer program product for evaluating an objectsuch as a wafer, a mask or reticles for micro-fabrication, flat paneldisplays, micro-electromechanical (MEMs) devices and the like during orafter manufacture.

According to an embodiment of the invention sub-areas are defined. Eachsub-area includes an array of repetitive structural elements. The arrayof repetitive structural elements is surrounded by at least one group ofnon-repetitive regions. Non-repetitive regions that belong to a singlegroup of non-repetitive regions are ideally identical to each other. Thenon-repetitive regions are arranged in a repetitive manner. Instead ofdid to die comparison the method, system and computer program productapply another comparison. This comparison is made between imageinformation of non-repetitive regions that belong to sub-areas (usuallyof the same die) that are proximate to each other. This type ofcomparison is more robust as it is less affected by process variations,color variations or changes in image acquisition conditions.

According to an embodiment of the invention instead of comparing imageinformation of the one structural element of an array of repetitivepatterns to image information of another structural element of the samearray of repetitive structural elements, image information of arepetitive structural element of a first sub-area is compared to imageinformation of a corresponding repetitive structural element of secondsub-area. This comparison overcomes comparison errors that result fromcomparing image information of structural elements that are located nearthe edge of an array of repetitive structural elements to imageinformation of structural elements that are located away from the edgeof the array of repetitive structural elements.

Yet according to another embodiment of the invention an array ofrepetitive elements is separated to an inner portion and to an outerportion. The outer portion can be located between the inner portion andthe non-repetitive regions that surround the array of repetitivestructural elements. The inner portion includes structural elements thatare selected so that their image information is not affected from thenon-repetitive regions while image information of structural elements ofthe outer portion can be affected by the non-repetitive regions. Imageinformation of a structural element within the inner portion of an arrayof repetitive structural elements is compared to image information ofanother structural element within the same inner portion. Imageinformation of structural elements within the outer portion of an arrayof repetitive structural elements that belongs to a certain sub-area arecompared to corresponding image information of structural elementswithin the outer portion of another array of repetitive structuralelements that belongs to another sub-area.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thus,these terms are not necessarily intended to indicate temporal or otherprioritization of such elements.

Unless states otherwise the term “corresponding structural elements”means structural elements that belong to different sub-areas that arepositioned at substantially the same position in relation to theboundaries of the sub-areas to which they belong.

The term “image information” means information that describesinformation obtained during the optical acquisition of an image.Typically this information include one or more gray level pixel but thisis not necessarily so. It is noted that the image information can beacquired by using imaging optics or non-imaging optics. Imageinformation can be digitally processed (for example by applying filters)before being compared to other image information.

FIG. 2 illustrates two sub-areas according to an embodiment of theinvention. Image information of a first sub-area is compared tocorresponding image information of a second sub-area FIG. 2 illustratesconsecutive sub-areas It is noted that image information comparison canbe made between sub-areas that are proximate to each other even if thesesub-areas are spaced apart from each other. Referring to the example setfourth in FIG. 1, a sub-area that includes array 10 and horizontalstitch 20 can be compared to another sub-area the includes array 18 andhorizontal stitch 27. It is noted that the image information of afirst-sub area can be compared to information representative of multipleother sub-areas. This can include a comparison to information of a firstsub-area to an average (or other statistical or on-statistical function)of multiple other sub-areas.

Two sub-areas can be regarded as proximate if the expected variationsbetween these areas due to process variation are below a certain processvariation threshold. Additionally or alternatively, two sub-areas can beregarded as proximate if the affect of color variations, temporalchanges in the optical image acquisition and/or illumination process oncomparison process is below a certain threshold. Each of thesethresholds can be set by a user, evaluation tool operator or objectmanufacturer in response to various parameters such as requiredsensitivity, required signal to noise ratio, expected defects to bedetected and the like. Typically sub-areas are regarded as proximate toeach other if the distance between these sub-areas does not exceed afraction of the length (or width) of a die.

FIG. 4 illustrates method 100 for evaluating an object according to anembodiment of the invention.

Method 100 starts by stage 110 of obtaining an image of an area of theobject; wherein the area includes multiple arrays of repetitivestructural elements that are at least partially surrounded by at leastone group of non-repetitive regions. Non-repetitive regions that belongto a single group of non-repetitive regions are ideally identical toeach other. Non-repetitive regions are arranged in a repetitive manner.

It is noted that the area can be much smaller than a die, can includeone or more dice, and the like.

Referring to the example set fourth in FIG. 2, the arrays of repetitivestructural elements are array 10 and array 13. The first group ofnon-repetitive regions include horizontal stitches 20, 23 and 26 whilethe second group of non-repetitive regions includes vertical stitches(denoted V).

Stage 110 can include optically (including using electron optics)obtaining these images, obtaining image information representative ofthese images and the like, it is noted that an image of an area, such asarea 8 of FIG. 1 can involve taking images of portions of area 8 andgenerating a synthetic image that includes portions of multiple separateoptically acquired images. It is further noted that the opticalacquisition of an image can require alignment. Alignment is typicallymuch easier (and less time consuming) when done per sub-area. It isnoted that optically obtained images of portions of an area canpartially overlap in order to compensate for various misalignmentproblems associated with the optically acquisition of images. It isfurther noted that the image of area 8 can be generated after acquiringmultiple images of that area and processing these images (for example—bysumming these images in order to average out random noise). Those ofskill in the art will appreciate that the image can be acquired by usingmonochromatic radiation, a charged particle beam, broadband radiation, apulsed light source, a continuous light source and the like.

Stage 110 is followed by stage 130 of providing an evaluation result inresponse to a comparison between image information of a first sub-areato image information of a second sub-area that is proximate to the firstsub-area. The first sub-area includes a first array of repetitivestructural elements and a first non-repetitive region. The secondsub-area includes a second array of repetitive structural elements and asecond non-repetitive region. It is noted that each sub-area can includeone or more portions of one or more non-repetitive regions.

Conveniently, stage 130 includes at least one stage out of stages 132,134 and 136.

Stage 132 includes comparing between image information of the firstsub-area and the image information of a second sub-area wherein thefirst sub-area includes a first array of memory cells and a firstnon-repetitive area includes a first stitch. The second sub-areacomprises a second array of memory cells and a second non-repetitivearea includes a second stitch. FIG. 2 provides an example of such subareas.

Stage 134 includes comparing image information of correspondingstructural elements of different sub-areas. Each structural element islocated at substantially the same position relative to boundaries of asub-area that includes the structural element. Stage 134 includes, forexample: (i) comparing image information of repetitive structuralelements that belong to a first sub area and are not located near theedge of their array to corresponding structural elements of a second subarea that are not located near the edge of another array, and (ii)comparing image information of structural elements of a non-repetitiveregion of a first sub area to corresponding structural elements ofanother non-repetitive region of a second sub area.

Stage 136 includes comparing between image information of structuralelements that are close to an edge of the first array and imageinformation of corresponding elements that are close to a correspondingedge of the second array. It is noted that stage 134 can be regarded asincluding stage 136 but this is not necessarily so.

FIG. 3 illustrates two sub-areas according to another embodiment of theinvention.

FIG. 3 illustrates that each array out of arrays 10 and 13 waspartitioned to an inner portion (10(1) and 13(1) respectively) and anouter portion (10(3) and 13(3) respectively). An outer region includesstructural elements that their image information is affected (at leastabove a certain threshold) from one or more non-repetitive region thatsurround its array. An inner region includes structural elements thattheir image information is not affected (or is affected below thecertain threshold) by a non-repetitive region. It is noted that thethreshold can represent a balance between the characteristics of cell tocell comparison and those of sub area to sub area comparison.

FIG. 5 illustrates method 200 according to an embodiment of theinvention.

Method 200 starts by stage 110 of obtaining an image of an area of theobject; wherein the area includes multiple arrays of repetitivestructural elements that are at least partially surrounded by at leastone group of non-repetitive regions; wherein non-repetitive regions thatbelong to a single group of non-repetitive regions are ideally identicalto each other; wherein the non-repetitive regions are arranged in arepetitive manner.

Stage 110 is followed by stages 144 and 146. These stages can beexecuted in parallel to each other but for simplicity of explanationFIG. 5 illustrates stage 144 as being followed by stage 146.

Stage 144 includes comparing image information of a first repetitiveelement that is included in an outer portion of the first array ofrepetitive structural elements to image information of a correspondingrepetitive element that is included in an outer portion of the secondarray of repetitive structural elements. The first array belongs to afirst sub area and the second array belongs to a second sub area. Thefirst and second sub areas are proximate to each other.

Stage 146 includes comparing image information of a first repetitiveelement that is included in an inner portion of the first array ofrepetitive structural elements to image information of anotherrepetitive element that is included in the inner portion of the firstarray of repetitive structural elements.

Stage 146 is followed by stage 148 of providing an evaluation result inresponse to at least one of the comparisons of stage 144 and 146.

FIG. 6 illustrates system 600 according to an embodiment of theinvention.

System 600 includes memory unit 620, processor 620 and image acquisitionunit 630. System 600 can optically obtain an image of an area or canreceive images (or information representative of such images) frominspection tools that include an image acquisition unit.

Memory unit 320 is adapted to store an image of an area of the object.The area includes multiple arrays of repetitive structural elements thatare at least partially surrounded by at least one group ofnon-repetitive regions; wherein non-repetitive regions that belong to asingle group of non-repetitive regions are ideally identical to eachother; wherein the non-repetitive regions are arranged in a repetitivemanner.

Processor 310 is adapted to provide an evaluation result in response toa comparison between image information of a first sub-area to imageinformation of a second sub-area that is proximate to the firstsub-area; wherein the first sub-area includes a first array ofrepetitive structural elements and a first non-repetitive region;wherein the second sub-area includes a second array of repetitivestructural elements and a second non-repetitive region.

Conveniently, processor 310 is adapted to compare between imageinformation of the first sub-area and the image information of a secondsub-area; wherein the first sub-area includes a first array of memorycells and a first non-repetitive area includes a first stitch; whereinthe second sub-area includes a second array of memory cells and a secondnon-repetitive area that includes a second stitch.

Conveniently, processor 310 is adapted to compare image information ofdifferent structural elements of different sub-areas; wherein eachstructural element is located at substantially the same positionrelative to boundaries of a sub-area that includes the structuralelement.

Conveniently, processor 310 is adapted to compare between imageinformation of structural elements that are close to an edge of thefirst array of repetitive structural elements and image information ofcorresponding elements that are close to a corresponding edge of thesecond array.

Conveniently, processor 310 is adapted to compare image information of afirst repetitive element that is included in an outer portion of thefirst array of repetitive structural elements to image information of acorresponding repetitive element that is included in an outer portion ofthe second array of repetitive structural elements.

Conveniently, processor 310 is adapted to compare image information of afirst repetitive element that is included in an inner portion of thefirst array of repetitive structural elements to image information ofanother repetitive element that is included in the inner portion of thefirst array of repetitive structural elements.

Conveniently, a computer program product is provided. It includes acomputer useable medium having a computer readable program, wherein thecomputer readable program, when executed on a computer, causes thecomputer to: receive an image of an area of the object; wherein the areacomprises multiple arrays of repetitive structural elements that are atleast partially surrounded by at least one group of non-repetitiveregions; wherein non-repetitive regions that belong to a single group ofnon-repetitive regions are ideally identical to each other; wherein thenon-repetitive regions are arranged in a repetitive manner; and providean evaluation result in response to a comparison between imageinformation of a first sub-area to image information of a secondsub-area that is proximate to the first sub-area; wherein the firstsub-area comprises a first array of repetitive structural elements and afirst non-repetitive region; wherein the second sub-area comprises asecond array of repetitive structural elements and a secondnon-repetitive region.

Conveniently, the computer program product causes the computer tocompare between image information of the first sub-area and the imageinformation of a second sub-area; wherein the first sub-area comprises afirst array of memory cells and a first non-repetitive area comprises afirst stitch; wherein the second sub-area comprises a second array ofmemory cells and a second non-repetitive area that comprises a secondstitch.

Conveniently, the computer program product causes the computer tocompare image information of different structural elements of differentsub-areas; wherein each structural element is located at substantiallythe same position relative to boundaries of a sub-area that comprisesthe structural element.

Conveniently, the computer program product causes the computer tocompare between image information of structural elements that are closeto an edge of the first array of repetitive structural elements andimage information of corresponding elements that are close to acorresponding edge of the second array of repetitive structuralelements.

Conveniently, the computer program product causes the computer tocompare image information of a first repetitive element, that isincluded in an outer portion of the first array of repetitive structuralelements to image information of a corresponding repetitive element thatis included in an outer portion of the second array of repetitivestructural elements.

Conveniently, the computer program product causes the computer tocompare image information of a first repetitive element that is includedin an inner portion of the first array of repetitive structural elementsto image information of another repetitive element that is included inthe inner portion of the first array of repetitive structural elements.

It is noted that although the above text refers to a comparison betweenimage information of structural elements of two sub areas that multiplecomparison between structural elements of more than two different subareas can be performed. For example, if there is a difference betweenimage information of two ideally identical corresponding structuralelements of two sub areas then at least one additional comparison can bemade in order to determine which structural element out of the first twothat were compared is defective.

It is further noted that the method, system and computer program productcan be applied mutatis mutandis to a comparison between imageinformation of a sub area to image information of a “golden” sub area.Thus, image information of a first sub area can be compared to imageinformation of a second sub area.

Those skilled in the art will readily appreciate that variousmodifications and changes can be applied to the embodiments of theinvention as hereinbefore described without departing from its scopedefined in and by the appended claims.

1. A method for evaluating a object; the method comprises: obtaining animage of an area of the object; wherein the area comprises multiplearrays of repetitive structural elements that are at least partiallysurrounded by at least one group of non-repetitive regions; whereinnon-repetitive regions that belong to a single group of non-repetitiveregions are ideally identical to each other; wherein the non-repetitiveregions are arranged in a repetitive manner; and providing an evaluationresult in response to a comparison between image information of a firstsub-area to image information of a second sub-area that is proximate tothe first sub-area; wherein the first sub-area comprises a first arrayof repetitive structural elements and a first non-repetitive region;wherein the second subarea comprises a second array of repetitivestructural elements and a second non-repetitive region, wherein saidcomparison comprises at least one of: a comparison of image informationof a first array of memory cells and a first non-repetitive areaincluding a first stitch and image information of a second array ofmemory cells and a second non-repetitive area that includes a secondstitch, a comparison of image information of different structuralelements of different sub-areas, each structural element being locatedat substantially a same position relative to boundaries of a sub-areathat comprises the structural element, a comparison of image informationof structural elements that are close to an edge of the first array ofrepetitive structural elements and image information of correspondingelements that are close to a corresponding edge of the second array ofrepetitive structural elements; a comparison of image information of afirst repetitive element that is included in an outer portion of thefirst array of repetitive structural elements and image information of acorresponding repetitive element that is included in an outer portion ofthe second array of repetitive structural elements; and a comparison ofimage information of a first repetitive element that is included in aninner portion of the first array of repetitive structural elements toimage information of another repetitive element that is included in theinner portion of the first array of repetitive structural elements.
 2. Asystem for evaluating an object; the system comprises: a memory unitadapted to store an image of an area of the object; wherein the areacomprises multiple arrays of repetitive structural elements that are atleast partially surrounded by at least one group of non-repetitiveregions; wherein non-repetitive regions that belong to a single group ofnon-repetitive regions are ideally identical to each other; wherein thenon-repetitive regions are arranged in a repetitive manner; and aprocessor adapted to provide an evaluation result in response to acomparison between image information of a first subarea to imageinformation of a second sub-area that is proximate to the firstsub-area, wherein the first sub-area comprises a first array ofrepetitive structural elements and a first non-repetitive region;wherein the second sub-area comprises a second array of repetitivestructural elements and a second non-repetitive region, wherein saidcomparison comprises at least one of: a comparison of image informationof a first array of memory cells and a first non-repetitive areaincluding a first stitch and image information of a second array ofmemory cells and a second non-repetitive area that includes a secondstitch, a comparison of image information of different structuralelements of different sub-areas, each structural element being locatedat substantially a same position relative to boundaries of a sub-areathat comprises the structural element, a comparison of image informationof structural elements that are close to an edge of the first array ofrepetitive structural elements and image information of correspondingelements that are close to a corresponding edge of the second array ofrepetitive structural elements; a comparison of image information of afirst repetitive element that is included in an outer portion of thefirst array of repetitive structural elements and image information of acorresponding repetitive element that is included in an outer portion ofthe second array of repetitive structural elements; and a comparison ofimage information of a first repetitive element that is included in aninner portion of the first array of repetitive structural elements toimage information of another repetitive element that is included in theinner portion of the first array of repetitive structural elements. 3.The system according to claim 2 comprising an image acquisition unitadapted to optically obtain an image of the area of the object.
 4. Acomputer program product comprising a computer useable medium having acomputer readable program, wherein the computer readable program, whenexecuted on a computer, causes the computer to: receive an image of anarea of the object; wherein the area comprises multiple arrays ofrepetitive structural elements that are at least partially surrounded byat least one group of non-repetitive regions; wherein non-repetitiveregions that belong to a single group of non-repetitive regions areideally identical to each other; wherein the non-repetitive regions arearranged in a repetitive manner; and provide an evaluation result inresponse to a comparison between image information of a first sub-areato image information of a second sub-area that is proximate to the firstsub-area; wherein the first sub-area comprises a first array ofrepetitive structural elements and a first non-repetitive region;wherein the second subarea comprises a second array of repetitivestructural elements and a second non-repetitive region, wherein saidcomparison comprises at least one of: a comparison of image informationof a first array of memory cells and a first non-repetitive areaincluding a first stitch and image information of a second array ofmemory cells and a second non-repetitive area that includes a secondstitch, a comparison of image information of different structuralelements of different sub-areas, each structural element being locatedat substantially a same position relative to boundaries of a sub-areathat comprises the structural element, a comparison of image informationof structural elements that are close to an edge of the first array ofrepetitive structural elements and image information of correspondingelements that are close to a corresponding edge of the second array ofrepetitive structural elements; a comparison of image information of afirst repetitive element that is included in an outer portion of thefirst array of repetitive structural elements and image information of acorresponding repetitive element that is included in an outer portion ofthe second array of repetitive structural elements; and a comparison ofimage information of a first repetitive element that is included in aninner portion of the first array of repetitive structural elements toimage information of another repetitive element that is included in theinner portion of the first array of repetitive structural elements.